A typical friction clutch operates in conjunction with the flywheel of an internal combustion engine and incorporates a cover, a pressure plate, and a driven plate. A spring assembly which may be a series of coil springs or a diaphragm spring acts between the cover and the pressure plate to grip frictionally the driven plate between friction faces of the flywheel and the pressure plate. A release mechanism is provided for releasing the friction grip between the pressure plate and the flywheel. With a typical diaphragm spring clutch the levers of the diaphragm spring, commonly referred to as fingers, act as part of the release mechanism by relieving the force of the diaphragm spring on the pressure plate. In the case of a coil spring clutch, the release mechanism is normally constituted by release levers which positively withdraw the pressure plate against the force of the coil springs.
In use the thickness of the driven plate decreases as a result of normal frictional wear with the result that the spring assembly has to move the pressure plate nearer to the flywheel and thus operates at a different point on its spring characteristic. Particularly in the case of a coil spring clutch this affects the engagement pressure between the pressure plate and the flywheel which in turn affects the torque capacity of the clutch. In the case of a diaphragm spring clutch, where a typical diaphragm spring has a non-linear characteristic, the clamp load between the pressure plate and the flywheel can remain almost constant over a considerable range of wear as the clutch wears but the clamp load can fall off rapidly with wear during the latter part of the life of the clutch. The load necessary to hold a disphragm spring clutch in its released position through the release mechanism can also increase with wear of the clutch.
These problems can be particularly significant in the case of twin plate clutches (i.e. clutches with two driven plates and an additional intermediate pressure plate between the two driven plates) because the extent of wear involved in two driven plates is greater than the corresponding wear in a single driven plate.
An automatic adjuster is often provided in a clutch operating mechanism. Such as adjuster does not affect the point of engagement of the clutch with respect to the clutch spring characteristic but merely adjusts the operating mechanism so that the usual clutch pedal engages or disengages the clutch at approximately the same position in its travel.
Also U.S. Pat. No. 3,938,636 describes a friction clutch which incorporates an automatic adjuster within the clutch pressure plate but this automatic adjuster is such that the two components of the pressure plate are disengaged from each other and re-engaged again with a wedging action on every clutch release operation and every clutch re-engagement operation. Such a mechanism is subject to wear in use so that it may have a short life and if it falls to operate, the whole clutch immediately becomes inoperative. Also the establishment of a wedging inter-engagement as required by every clutch operation and maintenance of a wedging engagement while the clutch remains engaged could become unreliable as it has to take place in what might be a vibratory environment with wide variations in temperature.
The present invention aims to overcome or substantially reduce the above disadvantages.